If you are creating your application instance from flask in file intro_to_flask and name of your instance variable is app:
intro_to_flask.py
...app = Flask()...
Change the Procfile to this
web: gunicorn intro_to_flask:app
The thing you need to point at is the app object, which as you show is inside the runserver file. So:
app
runserver
gunicorn runserver:app